moritz:
Really gotta go now, but just one more experiment: without recursion (actually, along the lines LanX has already posted).
>perl -wMstrict -le
"$_ = 'x77x888x1x';
;;
'zonk' =~ m{(\w+)}xms;
;;
print 'before: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
print X1();
print 'after: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
;;
sub X1 { s/(\d+)//; print qq{ '$1'}; return $1 + X2(); }
sub X2 { s/(\d+)//; print qq{ '$1'}; return $1 + X3(); }
sub X3 { s/(\d+)//; print qq{ '$1'}; return $1; }
"
before: $1 is 'zonk'
'77'
'888'
'1'
966
after: $1 is 'zonk'
Without recursion, value of $1 seems to be scoped to the subroutine 'block'.
No difference in results for Xn() + $1 versus $1 + Xn() or for $1 being undefined initially.